第6章 6.12

$z = f_{(x,y)}$ $x = a+bt$ $y = c+dt$

$x' = b$ $y' = d$

$\frac{dz}{dt} = b\frac{df}{dx} + d\frac{df}{dy}$

$\frac{d^2z}{dt^2} = b^2\frac{d^2f}{dx^2} + 2bd\frac{d^2f}{dxdy} + d^2\frac{d^2f}{dy^2}$